1. Field of the Invention
The present invention relates to an apparatus and method for writing servo tracks to a disk drive to reduce written-in runout.
2. Description of the Related Art
Disk drives generally can permanently store information in the form of magnetic transitions written onto and read from one or more rigid disks. These transitions are written by a recording head in generally concentric tracks on the disks. A servo system is provided in the disk drive that includes a plurality of servo sectors on the disks to enable the head to access, or to seek, a particular track. The servo system also enables the head to remain on the track, or to track-follow. Servo performance can degrade if the servo tracks written to the disk are non-circular, a phenomenon known as written-in runout.
One cause of written-in runout is disk flutter, a motion of the disk in a direction that is generally perpendicular to the disk surface, which is induced in the disk during the disk's rotation. Windage tends to be a prime cause of disk flutter, and varies proportionally to spindle speed. The flutter causes the surface of the disk to move axially and radially beneath the writing element of the recording head, which causes the writing element to write a track having a non-circular circumference around the disk. The non-circularity of a servo track is referred to as written-in runout of the servo track. When a servo track is read during track-following, the servo controller must position the reading element of the recording head so that the reading element follows the non-circular circumference of the servo track.
Typically, during a servo writing process, the spindle motor is rotated at a constant spindle motor speed to write all the servo tracks on the surface of the media. The particular spindle speed can be derived by running experiments prior to production to perform a series of measurements of disk runout at various speeds. The data from the measurements can then be summarized in what is known as a Campbell diagram, which reveals suitable spindle speeds for servo track writing. Generally, a single speed is chosen that is the highest spindle speed that does not generate greater than the maximum acceptable disk runout. The highest spindle speed is chosen because lower servo writing spin rates increase process time, which either limits disk drive production volume or forces disk drive manufacturers to purchase and maintain too many expensive servo writers.
As is well known, the density at which data bits are written on the disks is ever-increasing. The increased density requires the servo tracks to be written with less and less runout. At the same time, disk drive manufacturers are cutting costs and cannot increase the number of servo writers in order to write all servo tracks at lower speeds.